(1) Field of the Invention
The present invention relates to a color image forming apparatus of a laser scanning type wherein multiple laser scanning units are used to form color images based on a color digital image signal.
(2) Description of the Prior Art
In a color image forming apparatus, for example, in a color digital copier, the image of a color original input through its scanner is subjected to predetermined image processings and the image data which has been obtained by separating the input image into its color components and color conversion thereof, is reproduced for each of the colors so that the color printer portion produces a reproduced image of the color original. Further, recently, in order to achieve high-speed reproduction of color images at high resolution, color digital copiers which incorporate an image forming apparatus employing laser scan recording technology have been put on the market.
In these color digital copiers, multiple laser deflection scanning units are provided for individual recording portions so that the laser beam for each color, being modulated in accordance with the digital image signal, is brought to sweep the corresponding photoreceptor, thus forming a static latent image for image forming. Such a laser deflection scanning unit mainly comprises a polygon mirror and a motor for driving the polygon mirror at a high speed of rotation.
However, in this color digital copier, the polygon mirrors in the laser deflection scanning units are driven at high speed, which causes problems with respect to the motor's life and the problem of whirring due to air-turbulence caused by polygon mirror rotating etc.
To solve these problems, it is disclosed in Japanese Patent Publication Hei 4 No. 49,688, that when laser recording is not effected, the motors for rotating the polygon mirrors are retarded to a slower speed than when it is in operation or the motors are spun down with the drive of the motor completely stopped. However, in this configuration, the motors need to be controlled so as to completely recover the predetermined speed of rotation in a short period of time, in order to activate the laser recording portions, to revert from the waiting state back to the operating state in which the laser recording portions can effect image recording.
Further, in recent color digital copiers, a recording portion for BK(black) is generally provided in addition to Y(yellow), M(magenta) and C(cyan) recording portions in order to improve black reproduction and hence enhance the total color reproducibility of the color image. Therefore, color digital copiers have been put on the market which have the monochrome mode other than the color mode so as to allow for monochrome copying as well as color copying.
Also in a digital color copier of this type, if all the mirror-driving motors of the laser deflection scanning units in the recording portions are designed to rotate whenever a copying operation is to be done regardless of the mode, either color or monochrome mode, the mirror-driving motors for color will be also driven even in the monochrome mode. Resultantly, the problem of the life of the mirror-driving motors and their noise problem still remain unsolved.